1. In a small bowl, combine mayo, ketchup, BBQ sauce, mustard, and 1 Tbsp. dill pickle juice. Set aside.
2. Heat a large electric griddle, cast iron griddle or large frying pan to medium-high heat. If necessary, work in batches of 4, so there is ample space to smash/flatten the patties. Season the ground beef balls generously with salt and pepper.
3. Oil the pan, and place the balls of ground beef on the griddle. Using a large heatproof spatula, press down firmly on each ball, smashing it into a patty. Leave patties untouched on the griddle about 3 minutes, until the edges are starting to brown and caramelize. Apply firm pressure to each as they cook.
4. Flip the patties and place cheese on top immediately. Patties will likely need only another 2 minutes on second side. Remove from griddle, and set aside warm place. Drain the fat and wipe griddle to repeat with remaining patties if required.
5. If desired, butter the interior slices of the buns, and toast on a clean griddle about 30-60 seconds, rotating frequently until evenly browned and toasted.
6. To assemble, spread 1-2 tbsp. secret sauce on each half of the buns. Stack 2 patties on bottom bun, followed by sliced tomato, pickles, onion, shredded lettuce, and top bun.
